Axial fan assemblies, when utilized in an automotive application, typically include a shroud, a motor coupled to the shroud, and an axial fan driven by the motor. The axial fan typically includes a band connecting the respective tips of the axial fan blades, thereby reinforcing the axial fan blades and allowing the tips of the blades to generate more pressure.
Axial fan assemblies utilized in automotive applications must operate with high efficiency and low noise. However, various constraints often complicate this design goal. Such constraints may include, for example, limited spacing between the axial fan and an upstream heat exchanger (i.e., “fan-to-core spacing”), aerodynamic blockage from engine components immediately downstream of the axial fan, a large ratio of the area of shroud coverage to the swept area of the axial fan blades (i.e., “area ratio”), and recirculation between the band of the axial fan and the shroud. Other constraints factoring into the design include the material mass and cost of the shroud, overall stiffness of the shroud, especially in the motor stators securing the motor and fan to the shroud, and the overall volume occupied in the motor vehicle.
Prior axial fan assemblies have attempted to account for all of the above constraints to varying degrees of success. One prior art axial fan assembly 10 is illustrated in FIGS. 1A and 1B, and is representative of the fan assembly shown in U.S. Pat. No. 4,548,548. Of particular interest are the two radial gaps “g” formed between the shroud barrel 14 and the fan band 18, as well as the simple outlet downstream of the fan, which is formed by the cylindrical barrel shape. These features comprise the most common geometry used in the market. They provide for low material cost and low molding complexity, but also lower fan efficiency and higher fan noise than other outlets. The structural braces 22 shown are typically needed to stiffen the shroud 26 around the barrel 14 in order to transfer load from the motor stators 30 to the shroud 26. Even with the braces 22 shown, this design may also require additional bracing.
Another prior art axial fan assembly 40 is illustrated in FIGS. 2A and 2B, and is representative of the fan assembly shown in U.S. Pat. No. 5,489,186. This arrangement includes leakage stators 44 that reduce airflow recirculating around the fan band 18 as well as to remove tangential velocity from the re-ingested flow. The outlet bell 48 reduces loss in the wake. These features often result in higher fan efficiency and/or lower fan noise than for the design of FIGS. 1A, 1B. The structure comprised of outlet bell 48, leakage stator 44 and barrel 52 provides significantly more stiffness than the FIGS. 1A, 1B design. This design, however, requires more material and occupies more volume in the vehicle. The efficiency and noise of this design may not be as good as the other designs when combined with tight blockage from other automotive components situated downstream of the fan's outlet. This is due to its relatively high “aerodynamic depth” d, which causes more restriction of the fan wake impinging on the downstream blockage.
Yet another prior art axial fan assembly 60 is illustrated in FIGS. 3A and 3B, and is representative of the fan assembly shown in U.S. Pat. No. 7,762,769. This arrangement is a further refinement of the design shown in FIGS. 2A, 2B. Running clearances between the fan band 18 and the outlet bell 64 are provided by a radial gap “g”, rather than the axial gap. This allows smaller aerodynamic depth d. When in the presence of tight downstream blockage, this design results in less constriction in the fan wake impinging on the downstream blockage. Fan efficiency can thus be significantly higher than for the design of FIGS. 2A, 2B when compared in the presence of tighter downstream blockage. This outlet, however, tends to perform less effectively in the absence of downstream blockage, due to the radial gap “g” between the fan band 18 and the outlet bell 64. This design also provides stiffness comparable to the design of FIGS. 2A, 2B.